Embodiments of the present invention relate to electrostatic spray charging of conductive liquids, in particular to air-atomizing spray charging nozzles for conductive liquids that use the principles of induction or contact charging.
Electrostatic charging nozzles are well known and in widespread use in a number of commercial applications. Nearly every vehicle manufactured worldwide is painted electrostatically. Most of these industrial electrostatic spray systems charge spray by ionization and dispense powder or non-conductive liquids. There is a need for electrostatic spray devices that can reliably charge electrically conductive formulations, such as those that are water based. Several types of induction charging nozzles have been developed to produce electrostatically charged water sprays. U.S. Pat. No. 4,004,733 to Law shows an induction charging nozzle having a conductive ring surrounding a liquid jet inside a channel where high velocity air impacts the liquid stream, thereby creating a fine spray. Commercial versions of the nozzle described in the Law patent have been manufactured with deviations that include a liquid tip made from an insulating material, upstream grounding of the liquid, and lengthening the electrode to near the full length of the atomization channel. These modifications have made the nozzle of U.S. Pat. No. 4,004,733 reliable for use with water-based materials in most environments where the nozzle surfaces do not become excessively coated with conductive spray residue during a spraying operation. The conductive coatings on the surfaces of the nozzle can cause current leakage which reduces power supply voltage, damages surfaces, and reduces the internal charging field by elevating the voltage of the liquid stream.
Further patents to Cooper and Law, U.S. Pat. Nos. 5,704,554 and 5,765,761, utilize a fluid tip that is integral to the nozzle body, and utilize unique outside nozzle surface shapes to attempt to address some of the problems of stray electrical currents due to internal and external nozzle surface contamination. The fixed tip requires that the entire nozzle body be replaced in the event of mis-manufacturing, damage or wear, thereby increasing the cost and the effort of nozzle maintenance. The electrode portion of these nozzles is permanently pressed into the retaining cover. This does not allow replacement of the electrode alone—the entire cover assembly must be replaced. U.S. Pat. No. 4,343,433A to Sickles describes an induction charging nozzle with a fixed tip which utilizes air jets positioned around the main spray jet to prevent nozzle surfaces from becoming coated by spray. This method requires a significant amount of additional air energy, and the fixed tip and fixed electrode do not allow for adjusting for wear, machining tolerance, or replacing individual parts.
A series of electrostatic nozzle patents, U.S. Pat. Nos. 6,003,794, 6,138,922 and 6,227,466, to Hartman use an induction charging principle and liquid tip and air channel geometry that are similar to the above mentioned patents by Law, Cooper and Sickles. U.S. Pat. No. 6,003,794 describes nozzles having many components with stacked tolerances. These nozzles have a replaceable electrode but do not allow for adjustment. The nozzles mentioned in the above-identified patents charge well when made to precise, but expensive, machining tolerances, use matched components and are operated within a narrow range of liquid viscosities and liquid and air flow rates for a given internal spacing of components.
Variations in geometry of components causes charging variations which are due to improper droplet size or contact of the spray liquid with the walls of the induction electrode channel. Very small deviations in the internal spacing and dimensions of the atomization channel and liquid tip length have been observed to greatly diminish charging unless the air and liquid flows are within a narrow tolerance. These deviations occur due to nozzle manufacturing, from damage to components, and normal wear of components during use. Nozzle manufacturing deviations require that nozzle components be matched for optimal initial performance. This presents a problem since individual nozzle components wear over use and the entire nozzle often needs to be replaced with matching components. Measurements of spray charging from commercial versions of some typical nozzles with cost effective machining tolerances, but without using matched components, show over 30% variation from the same manufacturing run.
All of the above mentioned nozzles use air-atomizing induction-charging principles. With these nozzles the spray is charged to the opposite polarity as the electrode. Neither the liquid emitted from the tip nor the atomized spray is meant to contact the electrode. The advantage of such a system is that it produces high spray charging with very low electrode voltage and power. The disadvantage is that spray is attracted back to the nozzle surfaces. The wetted surfaces become conductive and reach the same polarity of the electrode, further attracting liquid spray droplets. The moisture deposits on the nozzle surface form into peaked shapes in response to the spray cloud space charge. The sharp points formed on these water droplets emit air ions that discharge large portions of the spray charge in the cloud. This effect can be minimized by adjusting the spray jet to a narrow column, using the air energy to force the spray a distance away from the nozzle. Another solution when this becomes a problem is to utilize contact charging principles. With contact charging types of nozzles the liquid stream is raised to a high voltage. This renders nozzle surfaces the same polarity as the spray cloud space charge and droplets are electrically repelled from the nozzle. The disadvantage is that the liquid container holding the spray liquid is also raised to high voltage, and as a result small containers should be used or isolation systems must be employed.
Operation of electrostatic charging nozzles in situations where contact with the nozzle by humans is possible, such as in applications of spray booths used for sunless-tanning, presents additional safety considerations in their design. One consideration is in limiting the exposure by humans to the electrode itself during operation. Another consideration is the reduction of the amount of leakage current from any portion of the nozzle where human contact could be made. The previously mentioned nozzles by Law and Cooper use an electrode which is embedded between layers of plastic or ceramic. This is an effective method for reducing the chance of direct contact with the electrode. However, commercial versions of the nozzle of U.S. Pat. No. 5,704,554 use an electrical contactor that is exposed when the cover is removed. This pointed contactor can be touched with the fingers and a shock can be received. The current from this contactor is in the range of 1 mA, capable of producing a shock intense enough to make the person involuntarily draw back very quickly and risk injury. Nozzles such as those described by Cooper and Law, Sickles, Hartman, and U.S. Pat. No. 4,664,315 to Parmentar et al. are induction charging devices and have the unfortunate characteristic of attracting spray back to the nozzle itself. This causes wetting of the nozzle face. Wetting by conductive liquids, near the jet outlet, can cause a conductive bridge to form to the electrode and cause shock when these forward nozzle surfaces are touched, even though the nozzle parts are made from insulating materials. The nozzle of Hartman, which is mounted with the electrode through a hole in a PVC tube structure, is particularly susceptible to leakage currents forward from the electrode. After a period of use black electrical tracking lines are evident on the surface of the tube. In addition the thin electrode cover may be easily removed during use causing direct exposure to the electrode.
Accordingly, there is a need for an air-atomizing charging nozzle for conductive liquids that has adjustable components to allow tuning for optimized spray quality and charging levels for a wide range of liquid viscosities and flow rates. It is desirable that the nozzle be manufactured with cost effective machining tolerances and not require component matching. It is also desirable that these tuning adjustments can be made while the nozzle is operating. It is also desirable that these adjustments remain set in place during normal nozzle operation. In addition, it is desirable to be able to easily replace and interchange nozzle components without adversely affecting charging and spray quality. Furthermore it is desirable to have the option to use the same nozzle as a contact charging device when necessary. Safety design considerations dictate that the nozzle have reduced leakage currents on all nozzle surfaces, particularly those interior and exterior surfaces which are easily touched by untrained operators.